Conventionally, in order to protect eyes from a strong light (UV-light), there have been proposed various contact lenses, or so-called photochromic contact lenses, having a function of light control (photochromism), in which the contact lenses are colored by a light and discolored by insulation of a light.
For example, JP-A-4-208919 discloses a method of manufacturing a photochromic hard contact lens by using a polymer obtained by polymerizing 30 to 80 parts by weight of methyl methacrylate, 2 to 30 parts by weight of bifunctional acrylate or methacrylate, and 1 to 50 parts by weight of additional acrylate-containing or methacrylate-containing monomer, under an existence of spiro naphthoxazine as a photochromic compound (coloring matter). In addition, JP-B2-4-28725 discloses a method of forming a contact lens from a resin block capable of exhibiting photochromism, in which the resin block is manufactured by polymerizing a mixture of methyl methacrylate or methyl methacrylate containing monomers and spiropyran compound capable of exhibiting photochromism. Further, Japanese Patent No. 2723225 discloses a technique for forming a photochromic contact lens from a polymer, wherein the polymer is obtained by using a photochromic compound of an ionic dissociation type, whose structure and color are changed by a light, which exhibits an ionic dissociation at its colored state, and has a copolymerizable functional group.
These conventionally known photochromic contact lenses are enabled to protect the lens wearer's eyes and exhibit a characteristic of antidazzling effect, by adding a compound (photochromic dye), which has a function of light control (photochromism), to the contact lens, or by introducing an unsaturated photochromic dye to a main chain of a polymer to form the contact lens. However, these background arts simply explain that the contact lenses are colored by an irradiation of the light, and if the light is insulated, the color is discolored immediately or without delay. There hasn't been proposed a method for improving the characteristic of discoloration and reducing the period of time for the discoloration.
If it takes too much time for discoloring the contact lens from a colored state to an uncolored state, this may cause problems. For instance, if the lens wearer is driving a car, visual acuity of objects is poor in a tunnel, which may cause undesirable effects for the driving. However, the discoloration of the conventionally used photochromic contact lens from the colored state to the uncolored state is not completed in a sufficiently short period of time. Therefore, there is desired a photochromic contact lens, which has an excellent discoloring characteristic and takes a short period of time for the discoloration.